With the recent advances in digital technology, radiographic images are converted into digital image signals, and image processing is performed for the digital image signals to display the resultant images on a display device such as a CRT or print them out. In radiography, in order to suppress the influence of X-rays on the outside of a necessary area (irradiation field area) and prevent scattering from the outside of the necessary area to prevent a decrease in contrast, an irradiation field stop is generally used to limit the irradiation field to the necessary area.
In general, an irradiation field area is extracted in advance to optimize an image processing parameter by eliminating unnecessary information from an image obtained by using an irradiation field stop.
Such irradiation field stops include rectangular and circular stops, and recognition processing suited to the shapes of various irradiation fields have been proposed.
For example, as processing for recognizing a rectangular stop, the method disclosed in Japanese Pat. Publication No. 05-049143 is available.
In this method, X- and Y-axes are set along two adjacent sides of the contour of a rectangular irradiation field, and image data are added/totaled along the X-axis and Y-axis directions.
In this case, the level of the added/totaled data within the irradiation field becomes higher than that of data in any other area outside the irradiation field, to which almost no X-rays are applied. Positions on the Y-axis at which the levels of the added/totaled data in the X-axis direction become higher than a predetermined threshold TH and positions on the X-axis at which the levels of the added/totaled data in the Y-axis direction become higher than the predetermined thumbnail TH are calculated. The rectangular area surrounded by lines which are located at the calculated positions on the Y-axis and extend in the X-axis direction and lines which are located at the calculated positions on the X-axis and extend in the Y-axis direction is set as an irradiation field area.
As processing for recognizing a circular stop, the method disclosed in Japanese Pat. Laid-Open No. 11-328372 is available.
According to this method, radiographic image data are scanned in an array manner or radially to detect differential signal values. Points at which the detected differential signal values are equal to or higher than a threshold TH and pixel signal values nearer to the image center become higher are detected as contour candidate points regarded as points located on the contour of the irradiation field. Of the plurality of detected contour candidate points, two points which are farthest from each other are extracted. In this case, since the straight line connecting the two points on the circular contour, the distance between which is largest, is regarded as the diameter of the circle, a circular area having the straight line connecting the extracted two points as a diameter is set as an irradiation field area.
As a method of recognizing an irradiation field independently of the shape of the irradiation field, the method disclosed in Japanese Pat. No. 02596744 is available. In this method, a plurality of edge candidate points located on the end portions of an irradiation field are calculated from predetermined points contained in radiation on the basis of differential values along a plurality of radial directions. The area defined by locally connecting these edge candidate points with straight lines is set as an irradiation field area.
According to the method disclosed in Japanese Pat. Laid-Open No. 05-049143, since the area surrounded by straight lines at positions where added/totaled data in the two set directions, i.e., the X and Y directions, are higher in level than the predetermined threshold TH is set as an irradiation field area, an irradiation field in a linear shape such as a rectangular shape can be extracted. However, this method cannot cope with an irradiation field in a polygonal shape constituted by straight lines in a plurality of directions or in a curved shape such as a circular shape.
In contrast to this, according to the method disclosed in Japanese Pat. Laid-Open No. 11-328372, since a circle having, as a diameter, a line connecting two points, of extracted contour candidate points, which have the maximum distance is used, a circular irradiation field can be extracted. Obviously, however, this method cannot cope with irradiation fields other than circular irradiation fields.
According to the method disclosed in Japanese Pat. No. 02596744, since the area defined by locally connecting a plurality of calculated edge candidate points with straight lines is set as an irradiation field, not only a rectangular irradiation field but also a polygonal or circular irradiation field can be extracted. However, even an irradiation field whose boundary line should be formed from a curved line, such as a circular irradiation field, is locally approximated by straight lines, and hence the irradiation field recognition result becomes a polygonal shape similar to a circular shape. That is, an irradiation field including a curved line, e.g., a circular irradiation field, can be recognized to some degree by polygonal approximation, but a curved shape cannot be extracted with high precision.
As described above, according to the prior art, the recognition precision is low in the recognition of irradiation field fields having different shapes.